Craig,
There are as many ways to pay a rep as there are ways to put ink on paper.
Everything is negotiable. Where I work (and have worked in the past) we have a system similar to what oxburger describes and it works ok. One thing we are looking at is a value added based commission. The higher the value added, the higher the commission paid. Keeps the bottom feeders from bringing in the wrong kind of work.
The real issue is finding the right rep that has the contacts to sell items in "your" sweet spot. My last company brought in a high end rep that sold only web work. (At the time we were only sheetfed). He failed and after 6 months, left for greener pastures. He couldn't transition his customer base or sales mentality to a sheetfed environment. Similar situation with a digital rep (thiswas 10-15 years ago). We knew we had to go there but didn't have the equipment to support the rep. Different outcome as that rep was able to shift into sheetfed mode until the company got the right equipment. He is still there selling to this day.
Short answer, use oxburger's method but be flexible. And I hope you find the right rep.
Good luck.
Greg